CAPS: 1997 Publications

We present data for the rate constant of the reaction NO2 + OH -> HONO2 in nitrogen from 2 to 600 torr at 300 K. This is the first application of our high-pressure flow technique to a pressure-dependent reaction. The pressure range in this experiment overlaps the ranges covered by traditional discharge flow studies and flash photolysis studies, allowing exploration of the transition between the low-pressure and intermediate pressure regimes. The measured rate constants are in excellent agreement with previously published values; however, current recommendations systematically overestimate the room temperature rate constant by between 10 and 30% in the pressure range 20 - 700 torr. A reanalysis of all available data yields a new recommendation in good agreement with mast of the data over the entire observational pressure range. This analysis includes an explicit treatment of the collisional efficiencies of different bath gases and includes the extreme broadening of the pressure falloff curve caused by the very strong HO-NO2 bond. We also report room temperature results for the reaction NO + OH over a more limited pressure range (4 to 75 torr). Our data agree with the currently recommended rate constants over this range. C1 HARVARD UNIV,DEPT CHEM,CAMBRIDGE,MA 02138. SUNY ALBANY,DEPT ATMOSPHER SCI,ALBANY,NY 12205. BASF AG,SCHWETZINGEN,GERMANY.

Gas-phase hydrogen (H) abstractions from molecules by free radicals have been studied extensively. They form the simplest class of elementary reactions and also play a key role in atmospheric chemistry and so are the centerpiece of models of reactivity. Despite intense scrutiny, two fundamental mechanistic issues remain unresolved: (1) Do H abstractions proceed directly or indirectly? (2) Do thermodynamic or electronic interactions determine their reaction barrier? The thermoneutral identity reaction, OH + H2O -> H2O + OH, provides an excellent opportunity to answer these questions, Several theoretically predicted H2O-HO complexes raise the possibility of an indirect mechanism, while no thermodynamic forcing influences the reaction barrier. To examine the various reactivity models, the isotopic scrambling reactions (OH)-O-18 + (H2O)-O-16 -> (H2O)-O-18 + (OH)-O-16 and (OD)-O-16 + (H2O)-O-16 -> (H2OD)-O-16 + (OH)-O-16 are studied in a high-pressure flow reactor. The measured rate constants are (2.3 +/- 1.0) x 10(-13) exp[-(2100 +/- 250)/T] cm(3) molecule(-1) s(-1) over the range 300-420 K ((2.2 +/- 1.0) x 10(-16) at 300 K) and (3 +/- 1.0) x 10(-16) cm(3) molecule(-1) s(-1) at 300 K, respectively. The similarity between the room temperature rates indicates a small secondary isotope effect. While the strong temperature dependence reveals that the predicted complexes do not stabilize the isotope exchange transition state sufficiently to bring its energy below the reactants, the small preexponential factor indicates that the complexes pose entropic constraints. Therefore, the reaction mechanism appears to be indirect. This is clarified by tracing the evolution of reagent electronic interactions and geometrical transformations along the reaction path. Activation energies of isotope exchange reactions are used to constrain the thermoneutral intercept for themodynamically based reactivity models. These thermochemical models are shown to be unreliable. However, a correlation between theoretical (ab initio) and experimental reaction barriers does capture gross reactivity trends. These measurements also exclude kinetic fractionation by OH as an important contributor to the isotopic fractionation of water in the earth's atmosphere. C1 HARVARD UNIV,DEPT CHEM,CAMBRIDGE,MA 02138.

A box model of the marine boundary layer is used to simulate the oxidation products of dimethylsulfide, including non-sea-salt (nss) sulfate, sulfur dioxide (SO2), methane sulfonic acid (MSA), dimethylsulfoxide (DMSO), methane sulfinic acid (MSEA), and dimethylsulfone (DMSO2), The gas phase oxidation schemes of Yin et al. [1990], Koga and Tanaka [1993], Hertel et al. [1994], Pham et al. [1995], and Benkovitz et al. [1994] are compared with field measurements using nine scenarios. Heterogeneous oxidation of SO2 in cloud droplets and sea-salt particles is also simulated. A sensitivity analysis is performed to evaluate which atmospheric parameters require the greatest attention in future field studies. Results indicate that the variations among the gas phase mechanisms are small with the parameterized mechanisms performing as accurately as the comprehensive ones. Among the nine scenarios tested, nss-sulfate is predicted without bias. Predicted MSA and SO2 concentrations depend more on the gas phase mechanism, with the mechanisms tending to underpredict SO2 concentrations. Compared to differences in MSA and SO2 predictions, DMSO, MSEA, and DMSO2 predictions by the various mechanisms are similar. Sulfate predictions are sensitive to the uncertain parameterizations of heterogeneous processes. The interaction of the marine boundary layer with the free troposphere can explain much of the discrepancy between the model predictions and measurements.

A dynamic model is developed for gas-particle absorptive partitioning of semi-volatile organic aerosols. The model is applied to simulate a pair of m-xylene/NOx outdoor smog chamber experiments. In the presence of an inorganic seed aerosol a threshold for aerosol formation is predicted. An examination of characteristic times suggests conditions where an assumption of instantaneous gas-particle equilibrium is justified. Semi-volatile products that are second-generation, rather than first-generation, products of a parent hydrocarbon cause a delay in aerosol formation due to the delayed rate at which the second-generation products are formed. The gas-particle accommodation coefficient is the principal transport parameter and is estimated to have a value between 1.0 and 0.1 for the m-xylene aerosol. (C) 1997 Elsevier Science Ltd. C1 CALTECH,DEPT CHEM ENGN,PASADENA,CA 91125. CALTECH,DEPT ENVIRONM ENGN SCI,PASADENA,CA 91125. CARNEGIE MELLON UNIV,DEPT CHEM ENGN,PITTSBURGH,PA 15213. CARNEGIE MELLON UNIV,DEPT ENGN & PUBL POLICY,PITTSBURGH,PA 15213.

A three-dimensional gas/aerosol atmospheric model is presented that predicts the size-resolved concentrations of all major primary and secondary components of atmospheric particulate matter (PM), including sulfate, nitrate, ammonium, chloride, sodium, elemental carbon, organic carbon, water, and crustal material. Aerosol size resolution is based on a sectional representation, typically extending from 0.01 to 10 mu m for aerosols and from 0.01 to 30 mu m when fog is present. The model is based on an internally mixed aerosol, wherein all particles in a specific size range are assumed to have the same chemical composition. Gas/aerosol equilibrium is computed based on the SEQUILIB algorithm of Pilinis and Seinfeld. An empirical fog model is included that approximates the effect of fogs on gas-phase photolysis rates, on aqueous-phase chemical reactions of sulfate and nitrate, and on the growth and shrinkage of the aerosol/fog droplet size distribution. The model is applied to simulate atmospheric conditions in the South Coast Air Basin of California during the 24-25 June 1987 episode of the Southern California Air Quality Study (SCAQS). The sensitivity of predicted aerosol levels to changes in source emissions is investigated. (C) 1997 Elsevier Science Ltd. C1 CALTECH,DEPT CHEM ENGN,PASADENA,CA 91125. CALTECH,DIV ENGN & APPL SCI,PASADENA,CA 91125. SONOMA TECHNOL INC,SANTA ROSA,CA 95403. UNIV DELAWARE,DEPT ENGN MECH,NEWARK,DE 19711. CARNEGIE MELLON UNIV,DEPT CHEM ENGN,PITTSBURGH,PA 15213. CARNEGIE MELLON UNIV,DEPT ENGN & PUBL POLICY,PITTSBURGH,PA 15213. ELECT POWER RES INST,PALO ALTO,CA 94303.

Submicron atmospheric particles that serve as cloud condensation nuclei (CCN) at low super-saturations are important for quantifying the effect of aerosols on cloud properties and global climate. In this study, we investigate experimentally the ability of model submicron aerosols consisting of pure organic species to become CCN at typical atmospheric supersaturations. The CCN activity of glutaric acid, adipic acid, and dyoctylphthalate (DOP) aerosols was determined by producing a nearly monodisperse distribution of submicron particles and comparing total CCN concentrations to total number concentrations. The measurements were performed using a Tandem Differential Mobility Analyzer in combination with a cloud condensation nuclei counter at supersaturations of 0.30 and 1.0%. The uncertainty in the measurements was determined by using NaCl and (NH4)(2)SO4 aerosols; the results indicated that activation diameters could be measured within an error of +/-16%. Adipic acid and glutaric acid aerosols served as CCN at both supersaturations and their behavior is in fair agreement with Kohler theory. On the other hand, DOP aerosol as large as 0.15 mu m in diameter, did not become activated, even at supersaturations as high as 1.2%. These results indicate that the CCN activity of hygroscopic organic aerosols may be comparable to that of some inorganic aerosols. (C) 1997 Elsevier Science Ltd.

We prove that the use of a cloud or fog droplet population's volume weighted average pH results in the underestimation of the actual rate of sulfate production for most atmospheric conditions. To quantify the magnitude of this error, we have developed two aqueous-phase chemistry models: a droplet size-resolved model and a bulk chemistry model. The discrepancy between the results of these two models indicates the magnitude of the error introduced by using bulk aqueous-phase properties. This error depends mainly on the availability of gas-phase species (SO2, O-3, H2O2, and NH3), the aerosol size/composition distribution, and the residence time of the air parcel in cloud containing air. The ratio of predicted sulfate production between the two models for the cases studied here varies from as low as unity to as high as 30. The largest ratios occur during the first few minutes of cloud formation. After this peak the difference in sulfate production rates between the two models decreases rapidly. For the scenarios simulated, the largest error introduced by the bulk modeling approach at the end of a cloud event was underprediction of the sulfate production by a factor of 2. The magnitude of the sulfate underprediction by the bulk model decreases with increasing initial levels of gas-phase NH3 and H2O2 and is rather insensitive to the gas-phase O-3 and SO2 concentrations.

To examine the effects of atmospheric pressure fluctuations on radon entry into houses, we report measurements of soil-gas and advective radon entry made using an experimental basement. Based on these measurements, we quantify the contribution of atmospheric pressure fluctuations, steady indoor-outdoor pressure differences, and molecular diffusion to the long-term radon entry rate into the experimental basement. In the absence of a steady indoor-outdoor pressure difference, atmospheric pressure fluctuations at the study site induce a radon entry rate 1.5 times greater than that due to molecular diffusion. A steady indoor-outdoor pressure difference reduces the contribution of atmospheric pressure fluctuations to the longterm radon entry rate. For sustained indoor-outdoor pressure differences with a magnitude greater than 1.5 Pa, atmospheric pressure fluctuations have essentially no effect on the time-averaged radon entry rate into the experimental structure. The results of this study demonstrate that under certain conditions, such as periods during which indoor-outdoor pressure differences are small, atmospheric pressure fluctuations will contribute measurably to the total radon entry rate into a building, potentially doubling indoor concentrations. However, in absolute terms, atmospheric pressure fluctuations drive approximately the same amount of entry as molecular diffusion and,therefore, will probably not cause houses to have long-term, elevated indoor radon concentrations. C1 UNIV CALIF BERKELEY,LAWRENCE BERKELEY LAB,DIV ENERGY & ENVIRONM,BERKELEY,CA 94720.

The seasonal variation in atmospheric transport patterns to Summit, Greenland, is examined using a 44-year record of daily, 10-day, isobaric back trajectories at the 500-hPa level. Over 24,000 modeled trajectories are aggregated into distinct patterns using cluster analysis. Ten-day trajectories reaching Summit are longest during winter, with 67% extending upwind (westward) as far back as Asia or Europe. Trajectories are shortest during summer, with 46% having 10-day origins over North America. During all seasons a small percentage (3-7%) of trajectories originate in west Asia/Europe and follow a meridional path over the Arctic Ocean before approaching: Summit from the northwest. Trajectories at the 700-hPa level tend to be shorter than at 500 hPa, with many of the 700-hPa trajectories from North America tracking over the North Atlantic and approaching Summit from the south. The long-range transport climatology for Summit is similar to a year-round climatology prepared for Dye 3, located 900 lan to the south [Davidson et al., 1993b]. An analysis of several aerosol species measured at Summit during summer 1994 reveals examples of the usefulness and also the limitations of using long-range air trajectories to interpret chemical data. C1 CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213. NOAA,CLIMATE MONITORING & DIAGNOST LAB,BOULDER,CO 80309. LAB GLACIOL & GEOPHYS ENVIRONM,F-38402 ST MARTIN DHER,FRANCE.

Behavioral experiments were performed on 342 subjects to determine whether behavior, which could affect the level of personal exposure, is exhibited in response to odors and labels which are commonly used for household chemicals. Potential for exposure was assessed by having subjects perform cleaning tasks presented as a product preference test, and noting the amount of cleaning product used, the time taken to complete the cleaning task, the product preference, and the exhibition of avoidance behavior. Product odor was found to affect product preference in the study with the pleasant odored product being preferred to the neutral and unpleasant products. Product odor was also found to influence the amount of product used; less of the odored products was used compared to the neutral product. The experiment also found that very few of the subjects in the study read the product labels, precluding analysis of the effect of such labels on product use. A postexperiment questionnaire on household cleaning product purchasing and use was administered to participants. The results indicate that significant gender-differences exist. Women in the sample reported more frequent purchase and we of cleaning products resulting in an estimated potential exposure 40% greater than for the men in the sample. This finding is somewhat countered by the fact that women more frequently reported exposure avoidance behavior, such as using gloves. Additional significant gender differences were found in the stated importance of product qualities, such as odor and environmental quality. This study suggests the need for further research, in a more realistic use setting, on the impact of public education, labels, and product odor on preference, use, and exposure for different types of consumer products.

Air-to-snow mineral transfer of crustal species on the Greenland Ice Sheet was studied at Dye 3 during a full annual cycle (August 1988-August 1989) and at Summit during a summer campaign (May 1991-September 1991). At Dye 3, continuously sampled aerosols (54 filters) show a clear seasonal cycle of insoluble mineral elements (Al, Fe, Ca) with strong concentration peaks in April. The simultaneous collection of fresh snows (32 precipitation events) reveals the same seasonal picture. Furthermore, a comparison of metal concentrations in both aerosol and snow indicates that the transfer of crustal elements (Fe or Al) from air to snow seems to occur without fractionation. This one year seasonal cycle Is recovered in snowpits excavated at Dye 3 (1 yr) and at Summit (3 yr) exhibiting no major post-depositional changes of crustal elements in aging snow. This suggests that the insoluble fraction of crustal elements, such as Fe or Al, in Arctic snows accurately reflects the seasonal atmospheric signal of mineral aerosols. (C) 1997 Elsevier Science Ltd. C1 OECD,ENVIRONM DIRECTORAT,F-75016 PARIS,FRANCE. UNIV PARIS 12,FAC SCI,F-94010 CRETEIL,FRANCE. UNIV PARIS 07,LAB INTERUNIV SYST ATMOSPHER,F-94010 CRETEIL,FRANCE. CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213. LAB GLACIOL & GEOPHYS ENVIRONM,F-38402 ST MARTIN DHER,FRANCE.